1. Field of the Disclosure
This invention relates in general to music systems. More specifically, this invention provides an arrangement to be used in connection with a musical instrument to create electrical signals in response to an input musical sound. These signals can be analog or digital and are adapted to be used with an electronic music synthesizer.
2. Description of the Prior Art
Electronic music synthesizers create varied musical sounds by generating various shaped wave forms at a desired pitch and amplitude. In general a synthesizer system will contain controls for varying the spectral content, harmonic content, amplitude, envelope shape, attack and delay time and other parameters that affect the timbre of musical. sounds as perceived by the human ear. The operator of an electronic synthesizer thus has two major functions that must be performed. He must "shape" the wave, thus determining its timbre and character, and he must input the note or notes that the shaped wave form should assume.
There are two basic ways to input this note information. One is to use a standard piano keyboard as the input device. The problem with this method is that the output signal cannot be dynamically controlled in response to the input signal. Specifically, the volume of the output signal will not depend on the force with which the key is depressed, and must be separately controlled. This utilization of a piano-type keyboard also limits the synthesizer operation to those who have the ability to play a keyboard instrument.
A more versatile method of providing "note" input is to use the musical signal from any musical instrument to control the synthesizer's sounds. Many advantages are obtained from this technique. For one, a much larger class of people could operate an electronic synthesizer, as any musical instrument with which one is familiar could be used as the input source. Another advantage is that the output volume of the note from the synthesizer can be made to depend on the volume of the input note from the instrument.
Any musical instrument that is able to generate musical vibrations can be used as an input source to an electronic synthesizer, providing an appropriate interface is used. Different synthesizers accept different input signals such as: linear DC control voltages proportional to the pitch of the desired note, sine waves representing the pitch to be output, or digital data to a microprocessor controlled music synthesizer. Since a musical sound typically includes a number of harmonics or overtones of varying amplitude, a problem has existed in that a synthesizer could falsely detect more than one frequency present in a single note. In response to this, a number of devices called pitch detectors or frequency followers have been proposed. The operation of a typical pitch detector will be described herein.
There are four basic methods of entering the musical signal into the pitch detector. One is by playing the instrument in proximity to an electromagnetic microphone. Mechanical transducers and electromagnetic pickups attached to the instrument itself can also be used, although this method is most often employed in conjunction with a stringed instrument. A fiber optic entry system can also be used. One form of fiber optic entry system, which is used exclusively with string instruments, detects the motion of the string and converts that motion to electrical signals. Another fiber optic system, as described in U.S. Pat. No. 4,442,750 to Bowley uses light modulation within optical fibers to generate a fiber optic signal which is later amplified.
Once an electrical signal corresponding to the musical sound has been entered into the pitch detector, a number of different methods can be used to extract the necessary information from the signal. U.S. Pat. No. 4,351,216 to Hamm discloses one form of electronic pitch detection system. In the Hamm pitch detector, a reference point in each cycle of the input signal is determined by setting a threshold level for the signal. This reference point is generated whenever the signal crosses this threshold level. An estimate of the period of the signal is obtained from the duration between successive reference points. In this system, a special algorithm must be used to determine a proper threshold level for each signal envelope.
U.S. Pat. No. 4,300,431 to de Rocco also teaches a pitch detector for an electronic musical instrument. This pitch detector generates a control voltage corresponding to the frequency of the input music signal, the purpose of this control voltage being to control an electronic music synthesizer. This system uses a closed loop in which a certain number of contiguous pitch values must be obtained before the pitch input is considered as detected. De Rocco deals with the problem of harmonics by using a low pass filter with a variable passband attempting to filter out the harmonics of the complex musical signal. The pulse train from the output of this filter runs a counter which generates a number proportional to the period of the pulse train. Using a shift register and a voltage controlled oscillator, an error voltage is obtained which is proportional to the frequency of the input signal and this error voltage is used to control the electronic synthesizer. A variation of this method of pitch detection is also used in U.S. Pat. No. 4,193,332 to Richardson. The disclosures of Hamm, de Rocco, and Richardson are expressly incorporated herein by reference.
U.S. Pat. No. 4,313,361 to Deutsch teaches a digital frequency follower which calculates using comparisons between internal test signals and the musical input signal to generate an indication of the pitch.
The present invention has as its objectives a new and improved method and apparatus for calculating the pitch and amplitude of an input complex musical signal. New functions can be performed with this system, and many of the problems existing in the prior art are overcome by the novel methods of this invention.
One major problem which still exists in the prior art is that of the long response time when a low note is entered into a pitch detector. This problem occurs because of the long period of the low note, and the necessity for a successive number of identical values to be sensed before a sufficient confidence level is obtained that the note being sensed is more than just spurious noise. Thus, with a low note, there is an appreciable delay time between the entry of the note and the control signal output in the prior art. This can make synchronization, which is necessary for a musical piece, difficult and disconcerting to the user. This problem is overcome by this invention as described with reference to the preferred embodiment.
Another problem in the prior art was that the necessity for the instrument to be in perfect tune. The pitch detector would sense the period of the note, and output a control signal corresponding to this period without any adjustments. This invention uses a method of quantizing each musical note to a predetermined period corresponding to a standard musical note before output of a control signal. In this way, the musical instrument used as the input device can be out of tune, but the output note will be perfectly quantized to a set musical reference note.
Other problems in the prior art are specific to interface devices used between stringed instruments and music synthesizers, such as the apparatus in the preferred embodiment. A number of such problems in the infra red pickup systems used on a stringed instrument are discussed in my co-pending application, Ser. No. 768,446 filed Aug. 22, 1985 . An Optical Pickup For Use With A Stringed Musical Instrument, the disclosure of which is expressly incorporated herein by reference.
Another problem in the prior art results from the physics of string motion. When a string is initially struck, the initial vibration is eccentric and unstable. Since one advantage of the current invention is rapid calculation of the pitch value, a special feature of this invention allows it to detect the pitch of the string during this initial unstable period.
One known method of pitch detection relies on low pass filtering of the musical note to remove higher order harmonics from the musical signal. A problem in the prior art is that the musical range of a single string can be two octaves or more, and this cannot be efficiently accommodated with a single low pass filter. A special circuit in this invention consisting of two low pass filters which function in a special way overcomes this limitation.
Another problem in the prior art is that of the pitch detector obtaining an erroneous pitch value immediately after obtaining a valid one. One reason for this is that when a string is released by a player, there is a brief period during which a pitch one-half step lower than the note is sounded. The microprocessor in the present invention detects such a spurious pitch and eliminates the incorrect number value being generated. Another incorrect pitch value can be obtained when an octave harmonic appears to the pitch detector. The controlling program of this system will not output an octave value unless the string or note is retriggered, thus also eliminating octave errors.
The present invention relates to a microprocessor controlled pitch and amplitude calculator and converter for use with any source of musical sound input. By use of a number of novel methods and designs, this system overcomes many of the limitations of the prior art as listed above.